Fix DCB closing debug assertion

The assertion for a DCB with a fd of -1 needs the internal DCBs as well
since they do not have valid file descriptors.

server/core/dcb.cc

@@ -1277,7 +1277,7 @@ void dcb_final_close(DCB* dcb)
         else
         {
             // Only listeners are closed with a fd of -1
-            mxb_assert(dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER);
+            mxb_assert(dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER || dcb->dcb_role == DCB_ROLE_INTERNAL);
         }
 
         dcb->state = DCB_STATE_DISCONNECTED;